1829, John Wesley, Sermon LVIII: On the Eternity of God, in Sermons on Several Occasions, Volume 2, 10th edition, page 1,

Eternity has generally been considered as divisible into two parts; which have been termed, eternitya parte ante, and eternitya parte post: that is, in plain English, that eternity which is past, and that eternity which is to come.

1886, Augustus Hopkins Strong, Systematic Theology: a Compendium and Commonplace-book Designed for the Use of Theological Students‎[1], page 190:

Those who like not the company of the saints on earth will get none of it in eternity; but, as godless company is their delight now, they will afterwards get enough of it, when they have eternity to pass in the roaring and blaspheming society of devils and reprobates in hell.

We sometimes speak of eternity in contradistinction to time; and often say, "through time and into eternity;" and again "from eternity to eternity," which is simply another form of expressing the same idea, and "pass through time into eternity." in other words, time is a short period allotted to man in his probationary state—and we use the word time in contradistinction to the word eternity, merely for the accommodation of man in his finite sphere, that we may comprehend and learn to measure periods.

(countable) A period of time which extends infinitely far into the future.

In the sense "a comparatively long time", eternity is always used with the indefinite article (an eternity).

In philosophy, the common use of eternity to refer to an infinite time is considered incorrect, eternity referring to existence outside of time; existence within time but of an infinite temporal duration is called everlastingness or sempiternity

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